Inkjet recording apparatus having an electrostatic actuator and method of driving it

Abstract: Described are an inkjet recording apparatus and a method of controlling it. The apparatus comprises an inkjet head (10) having for each of one or more nozzles (4), an ink passage in communication with the nozzle, and an electrostatic actuator comprising a pair of capacitor plates of which one plate is formed by or attached to a diaphragm provided in a part of said ink passage, while the other plate is formed by an electrode disposed outside of said ink passage in opposition to the diaphragm with a gap therebetween, and drive means (210, 212, 213; 210) for selectively charging and discharging each actuator such as to displace its diaphragm by an electrostatic force, thereby to eject ink droplets from said one or more nozzles. To avoid an adverse effect of a residual charge remaining on the actuator after it has been discharged, in one embodiment a first voltage is applied to charge an actuator and a second to reset an actuator. The second voltage is of a polarity opposite to that of the first voltage such as to remove the residual charge. In an alternative embodiment the actuator is preset by applying a second voltage of a magnitude equal to or greater than the maximum magnitude of the first voltage to be expected. Both measures ensure a stable actuator operation resulting in a good recording quality.

Description: The present invention relates to an inkjet recording apparatus having a so-called ink-on-demand type inkjet head which ejects ink droplets only when respective dots are actually to be recorded on a recording medium. More specifically, the invention relates to an inkjet recording apparatus having an electrostatically driven inkjet head and to a method of driving it. Mainly two kinds of ink-on-demand type inkjet heads are currently used differing in the way of generating the pressure required for ink ejection. One kind uses piezoelectric actuators for this purpose as disclosed in, e.g., DE-A-31 47 107 and EP-A-0 337 429, while the other employs heating elements for heating ink so as to generate bubbles as described in, e.g., JP-B-59911/1986. Each of these two kinds of inkjet head has its own merits and demerits. While the former type suffers from problems in manufacturing when a certain nozzle density and precision is required, it enjoys a high reliability and a long service life. On the other hand, the bubble type inkjet head presents less manufacturing problems but its resistive heating elements tend to become damaged over time as a result of the repeated rapid heating and cooling and the impacts caused by collapsing bubbles, and so the practical service life of the inkjet head is accordingly short. Thus, none of these two kinds of inkjet heads is really fully satisfactory. JP-A-24218/1990 discloses a method of driving an on-demand type inkjet head of the above mentioned kind using piezoelectric actuators. According to this drive method, during the printer standby state, an electrical pulse is applied to the piezoelectric actuator in the same direction as the polarization voltage of the actuator, thereby charging the actuator and reducing the volume of an ink or pressure chamber. To eject an ink droplet during printing, first, the actuator is gradually discharged to increase the volume of the pressure chamber, and then an electrical pulse is again applied to the actuator to rapidly charge it and to decrease the pressure chamber volume, thereby ejecting ink from a nozzle. To eject the ink with greatest efficiency at a low drive voltage level, the time for applying the electric pulse after having discharged the actuator is selected to coincide with the peak value of a damped vibration the ink supply system undergoes when in response to the discharging of the actuator ink is suctioned into the pressure chamber. This conventional drive method is one of the best methods available for inkjet heads using a piezoelectric actuator. A third known principle for pressure generation in an inkjet head makes use of an electrostatic force, i.e., employs an electrostatic actuator as disclosed in JP-A-289351/1990 and US-A-4,520,375. More particularly, JP-A-289351/1990 discloses an inkjet head comprising a silicon substrate having formed therein ink passages each connected to a respective nozzle at one end and to a common ink reservoir at the other end. A side wall portion of the ink passage is formed by a diaphragm as a vibration plate. A respective individual or nozzle electrode is provided on the outside surface of each diaphragm. Disposed opposite the nozzle electrodes, via a gap, is a common electrode. Each diaphragm with its nozzle electrode and the opposing common electrode constitutes an electrostatic actuator including a capacitor formed by the nozzle electrode, the common electrode and an insulator therebetween. A similar electrostatic actuator or fluid jet ejector is disclosed in US-A-4,520,375. In this latter prior art, by utilizing its semiconducting property, the thin silicon diaphragm itself forms one electrode of the capacitor. Impressing a time varying voltage on the capacitor causes the diaphragm to be set into mechanical motion and the fluid to exit responsive to the diaphragm motion. The electrostatic principle utilized in this prior art offers advantages such as compactness high density, and a long service life, and, therefore, appears to be a promising alternative by which the above stated problems of the prior art using either piezoelectric actuators or heating elements may be resolved. The document EP-A-0 479 441 discloses an inkjet recording apparatus according to the precharacterizing clause of claim 1. In this prior art drive means include an oscillation circuit for alternately generating a zero voltage and a positive voltage or an AC electric source to selectively drive each actuator in response to a print control signal. In an alternative embodiment EP-A-0 479 441 discloses a structure of an electrostatic actuator in which the diaphragm itself does not form one of two capacitor electrodes to which the drive voltage is applied. Instead, two separate electrodes are disposed side by side. One or both electrodes are located opposite the diaphragm and positive and negative pulses are alternately applied to these two electrodes, one pulse for attracting the diaphragm to prepare for ejection of an ink droplet and the following pulse of opposite polarity for repulsing the diaphragm and to effect the ejection of the ink droplet. However, a practical implementation of an ink-on-demand type inkjet head using such electrostatic actuators for pressure generation and featuring high quality printing and constant high efficiency has not been possible yet. The results that were achieved when the drive method described above for piezoelectric actuators was applied in an inkjet head using an electrostatic actuator were not satisfactory. More particularly, the inkjet head using electrostatic actuators suffers from an unstable, partly insufficient ink ejection volume and a reduced printing speed, i.e. low reliability and insufficient print quality. Therefore, it is an object of the present invention to provide an inkjet recording apparatus having an electrostatically driven inkjet head as well as a drive method for such apparatus that allow to achieve a reliability and a recording quality as least as good as those achieved with conventional recording apparatus either of the piezoelectric type or the bubble type. This object is achieved with an inkjet recording apparatus as claimed in claim 1 and a method as claimed in claim 8, respectively. Preferred embodiments of the invention are subject-matter of the dependent claims. The present invention is based on the recognition that in an electrostatic actuator in which a diaphragm and a nozzle electrode form a pair of capacitor plates with a dielectric therebetween, when a voltage is applied to charge the capacitor and the capacitor is subsequently discharged there remains a polarization in the dielectric . The electric field established by this residual charge decreases the relative displacement of the diaphragm and the nozzle electrode. This decrement in the relative displacement is a cause of insufficient ink ejection volume and reduced printing speed, resulting in a low print quality, such as low density and pixel shifting, and in a lower reliability because of dropouts. In addition, the magnitude of this residual charge tends to vary according to the history of past applied voltages. As a result, the relative displacement of the diaphragm and individual electrodes is indefinite and unstable, further contributing to a low recording quality and reliability. The invention removes, or at least substantially reduces, the adverse effect of the residual diaphragm displacement caused by the residual charge of the electrostatic actuator by either removing the residual charge (resetting the actuator) or by bringing it into a definite state (presetting the actuator). For resetting the actuator a voltage of a polarity opposite to the normal drive voltage is applied while for presetting a voltage equal to or greater than the maximum expectable drive voltage is applied at predetermined times. Preferred embodiments of the present invention will be described in detail below with reference to the drawings, in which: Fig. 1 is a block diagram of a printer according to a first preferred embodiment of the invention. Fig. 2 is a partially exploded perspective view of the inkjet head of a preferred embodiment of the invention, Fig. 3 is a side cross-section of the inkjet head shown in Fig. 2, Fig. 4 is a sectional view from line A-A in Fig. 3, Fig. 5 is a schematic diagram illustrating the charge distribution in the diaphragm and the nozzle electrode arrangement when a voltage whose polarity is selected in accordance with the invention is applied, Fig. 6 is a diagram similar to Fig. 5 showing the polarization in the dielectric between the diaphragm and the nozzle electrode, Fig. 7 is a diagram corresponding to Fig. 6 and illustrating the residual charge that remains after discharging, Fig. 8 schematically shows the deflection of the diaphragm (a) in an initial state with no voltage applied and no residual charge, (b) with a voltage applied and (c) discharged but still having a residual charge, Fig. 9 is a schematic diagram of the drive control circuit used in a preferred embodiment, Fig. 10 is a conceptual diagram of a printer embodying the invention, Fig. 11 is a flow chart illustrating a method of controlling an inkjet printer as shown in Fig. 1, Fig. 12 is a flow chart of the subroutines shown in Fig. 11, Fig. 13 is a timing chart for explaining the method illustrated in the Fig. 11 flow chart, Fig. 14 is a flow chart of an alternative control method of an inkjet printer as shown in Fig. 1, Fig. 15 is a flow chart of a subroutine shown in Fig. 14, Fig. 16 is a timing chart for explaining the alternative control method, Fig. 17 is a flow chart of another alternative control method of an inkjet printer as shown in Fig. 1, Fig. 18 is a flow chart of the subroutines shown in Fig. 17, Fig. 19 is a block diagram of a printer according to a second embodiment of the invention, Fig. 20 shows the deflection of the diaphragm under various conditions, Fig. 21 is a graph showing how the ink ejection speed at a constant drive voltage (38 V) varies with the drive voltage applied in the preceding period, Fig. 22 is a schematic diagram of the drive control circuit for the inkjet head of the second embodiment, Fig. 23 is a flow chart illustrating a method of controlling a printer as shown in Fig. 19, Fig. 24 is a flow chart illustrating an alternative control method of a printer as shown in Fig. 19, and Fig. 25 is a flow chart of the subroutines shown in Fig. 24. Fig. 2 is a partially exploded perspective view and cross-section of a preferred embodiment of the inkjet head of a recording apparatus embodying the present invention. Note that while this embodiment is shown as an edge type head wherein ink is ejected from nozzles provided at the edge of a substrate, the invention may also be applied to a face type head wherein the ink is ejected from nozzles provided on the top surface of the substrate. Fig. 3 is a side cross-section of the assembled inkjet head, and Fig. 4 is a sectional view from line A-A in Fig. 3. The inkjet head 10 of this embodiment is made up of three substrates 1, 2, 3 one stacked upon the other and structured as described in detail below. A first substrate 1 is sandwiched between second and third substrates 2 and 3, and is made from a silicon wafer. Plural nozzles 4 are formed between the first and the third substrate by means of corresponding nozzle grooves 11 provided in the top surface of the first substrate 1 such as to extend substantially in parallel at equal intervals from one edge of the substrate. The end of each nozzle groove opposite said one edge opens into a respective recess 12. Each recess in turn is connected via respective narrow grooves 13 to a recess 14. In the assembled state the recess 14 constitutes a common ink cavity 8 communicating via orifices 7 formed by the narrow grooves 13, and ink chambers 6 formed by the recesses 12 with the nozzles 4. In the present embodiment, each orifice 7 is formed by three parallel grooves 13 mainly to increase the flow resistance but also to keep the inkjet head operative if one of the grooves becomes clogged. Electrostatic actuators are formed between the first and the second substrate. The bottom of each ink chamber 6 comprises a diaphragm 5 formed integrally with the substrate 1. As will be understood, the grooves and recesses referred to above can be easily and precisely formed by photolithographic etching of the semiconductor substrate. A common electrode 17 is provided on the first substrate 1. The magnitude of the work function of the semiconductor forming the first substrate 1 and the metal used for the common electrode 17 is an important factor determining the effect of electrode 17 on first substrate 1. The semiconductor material used in this embodiment has a resistivity of 8 - 12 Ωcm, and the common electrode 17 has in fact a two-layer structure made from platinum on a titanium base layer or gold on a chrome base layer, the latter being provided mainly to improve the bonding strength between the substrate and the electrode. The present invention shall not be so limited, however, and various other material combinations may be used according to the characteristics of the semiconductor and electrode materials. Borosilicate glass, such as Pyrex glass, is used for the second substrate 2 bonded to the bottom surface of first substrate 1. Nozzle electrodes 21 are formed on the surface of second substrate 2 by sputtering gold to a 0.1 µm thickness in a pattern essentially matching the shape of diaphragms 5. Each of nozzle electrodes 21 comprises a lead member 22 and a terminal member 23. A 0.2 µm thick insulation layer 24 for preventing dielectric breakdown and shorting during inkjet head drive is formed from a Pyrex sputter film on the entire surface of the second substrate 2 except for the terminal members 23. In addition or as an alternative to the insulation layer 24 an insulation layer (26 in Fig. 5) may be provided on the side of the diaphragms 5 facing the nozzle electrodes. Since the diaphragms 5 consist of a semiconductor material such insulation layer may be easily formed to a thickness of 0.1µm to 0.2µm by oxidizing the semiconductor material. Such oxide insulation layer exhibits excellent mechanical strength, insulation performance and chemical stability and substantially reduces the possibility of a dielectric breakdown in case of a contact between the diaphragm and the nozzle electrode. This is an advantage of using the semiconductor material itself as an electrode of the electrostatic actuator. A recess 15 for accommodating a respective nozzle electrode 21 is provided below each diaphragm 5. Bonding the second substrate 2 to the first substrate 1 results in vibration chambers 9 being formed at the positions of recesses 15 between each diaphragm 5 and the corresponding nozzle electrode 21 opposite to it. In this embodiment, recesses 15 formed in the bottom surface of the first substrate 1 provide for gaps between the diaphragms and the respective electrodes 21. The length G (see Fig. 3; hereinafter the "gap length") of each gap is equal to the difference between the depth of recess 15 and the thickness of the electrode 21. It is to be noted that this recess can be alternatively formed in the top surface of the second substrate 2. In this preferred embodiment, the depth of recess 15 is 0.6 µm, and the pitch and width of nozzle channels 11 are 0.72 mm and 70 µm, respectively. As with second substrate 2, borosilicate glass is used for the third substrate 3 bonded to the top surface of first substrate 1. Bonding third substrate 3 to first substrate 1 completes formation of nozzles 4, ink chambers 6, orifices 7, and ink cavity 8. An ink supply port 31 is formed in third substrate 3 so as to lead into ink cavity 8. Ink supply port 31 is connected to an ink tank (not shown in the figure) using a connector pipe 32 and a tube 33. First substrate 1 and second substrate 2 are anodically bonded at 300°C to 500°C by applying a voltage of 500 V to 800 V, and first substrate 1 and third substrate 3 are bonded under the same conditions to assemble the inkjet head as shown in Fig. 3. After bonding the substrates, gap length G between diaphragms 5 and nozzle electrodes 21 is 0.5 µm in this embodiment. The distance G1 between diaphragms 5 (or the insulation layer 26, if any) and insulation layer 24 covering nozzle electrodes 21 is 0.3 µm. The thus assembled inkjet head is driven by means of a drive unit 102 connected by leads 101 to common electrode 17 and terminal members 23 of nozzle electrodes 21. Drive unit 102 includes a plurality of drive circuits (213 in Fig. 9, 413 in Fig. 22), one for each actuator. Ink 103 is supplied from the ink tank (not shown in the figures) through ink supply port 31 into first substrate 1 to fill ink cavity 8 and ink chambers 6. Also shown in Fig. 3 is an ink droplet 104 ejected from nozzle 4 during inkjet head drive, and recording paper 105. The electrical connections of the present embodiment are described next. Due to the MIS structure formed between the nozzle electrode, the insulating gap between the nozzle electrode and the diaphragm, and the diaphragm itself, there may be a large difference in the current value depending on the polarity of the applied voltage because of the effect of the space-charge layer. When the semiconductor used for the substrate is p-type silicon, the substrate acts as a conductor when, relative to the nozzle electrodes 21, a positive potential is applied to the common electrode 17, but when a negative potential is applied, the substrate does not act as a conductor and instead a space-charge layer is created. Fig. 5 is a schematic view illustrating the distribution of electric charges in the diaphragm and the nozzle electrode when the polarity of the applied drive voltage is selected in accordance with a preferred embodiment of the present invention. A p-type silicon is used for first substrate 1 in this embodiment and the common electrode 17 and the nozzle electrodes 21 of the electrostatic actuators are connected to drive circuits (213 in Fig. 9, 413 in Fig. 22) so that for charging an actuator a pulse voltage is applied by which the common electrode is rendered positive with respect to the nozzle electrode 21. The p-type silicon is doped with acceptor impurities such as boron and has as many holes as the number of acceptor atoms. The pulse voltage establishes an electrostatic field directed from the diaphragm to the nozzle electrode. Because of this field the holes 19 in the p-type silicon migrate towards insulation layer 26 leaving negatively charged acceptor ions. Because holes are injected from the common electrode 17 the negative charge of the acceptor ions is neutralized. Therefore, the diaphragm assumes a positive charge with no space-charge layer being created, i.e. the diaphragm or the first substrate functions as a conductor. In addition, a negative charge accumulates on the nozzle electrodes 21 side. As a result, the pulse voltage applied between a diaphragm 5 and its opposing nozzle electrode 21 generates an attractive force, due to static electricity, sufficient to deflect diaphragm 5 towards the nozzle electrode 21. The occurrence of residual charges in the electrostatic actuator will be explained with reference to Fig. 6 and Fig. 7 which illustrate the capacitor structure forming the actuator. Fig. 6 shows the state when a voltage of a certain value is applied and the capacitor is charged, and Fig. 7 shows the state when subsequently the capacitor is discharged through a discharge resistor 46. In both Figs. 6 and 7, diaphragm 5 is made from a semiconductor and common electrode 17 is the above mentioned metal forming an ohmic contact with the semiconductor, and diaphragm 5 is coated by insulation layer 26. Insulation layer 24 formed on nozzle electrode 21 is opposite insulation layer 26 across gap 16, and insulation layer 26, gap 16, and insulation layer 24 together form an insulator structure or dielectric 27 inside the parallel flat capacitor formed by diaphragm 5 and nozzle electrode 21. When a voltage is applied to the capacitor, the dielectric produces polarization 28 in a direction to cancel the field E generated by the applied voltage as shown in Fig. 6. Upon switching the capacitor from charging to discharging most of the polarization 28 dissipates within a short time. The delay time from the moment when the discharge is started to the dissipation of the polarization is called the relaxation time, and varies greatly with the type of polarization. When the dielectric (insulation layer) between diaphragm 5 and nozzle electrode 21 of the preferred embodiment is polarized, polarization components known, for example, as ion polarization and interfacial polarization, and having a relatively long polarization relaxation time are involved in addition to short relaxation time atomic polarization and electron polarization. Ion polarization occurs as a result of Na+, K+, and/or B+ in the insulation layer travelling along the generated field; interfacial polarization occurs from movement at crystal interfaces within the dielectric. Thus, part of the polarization remains as a result of repeated voltage application or extended continuous application, and the dielectric (24, 26) between diaphragm 5 and nozzle electrodes 21 of the embodiment retains partial polarization for an extended period as shown in Fig. 7. The dielectric body thus effectively contains residual polarization 29, and the residual field P produced by the residual polarization remaining between diaphragm 5 and nozzle electrode 21 causes a small residual relative displacement of diaphragm 5 and nozzle electrode 21. Fig. 8 illustrates three different states of the actuator schematically represented by only the diaphragm 5 and the nozzle electrode 21. Fig. 8 (a) shows the state when a voltage has not yet been applied to the capacitor formed by diaphragm 5 and nozzle electrode 21: as shown in the figure, the diaphragm 5 and the nozzle electrode 21 are parallel. Fig. 8 (b) shows the state when a voltage is applied and the capacitor charged: as shown in the figure, diaphragm 5 deflects. This deflection will be referred to below as ΔV1. Fig. 8 (c) shows the state after a subsequent discharge of the capacitor. Despite the discharged state diaphragm 5 remains deflected by the residual field explained above; this deflection will be referred to as ΔV2 below. The relative displacement of diaphragm 5 and nozzle electrode 21, i.e. the displacement utilized for ink ejection, is therefore only ΔV1 - ΔV2, demonstrating the drop in relative displacement. As described above, this decreased relative displacement of diaphragm 5 and nozzle electrode 21 is a cause of reduced ink ejection volume, ink speed, and other ink ejection-related defects, and thus adversely affects inkjet printer reliability and print quality. Having thus described the preferred structure of and the problems involved in an electrostatically driven inkjet head for a recording apparatus, a first embodiment of the invention suitable to avoid those problems will be explained next. In this embodiment, in order to remove the adverse effect of the residual charge causing a residual deflection of the diaphragm, a voltage of a polarity opposite to that shown in Fig. 6 is applied between diaphragm 5 and nozzle electrode 21 to cancel the residual charge and, thus to reset the actuator. Fig. 1 is a block diagram of an inkjet printer as a preferred embodiment of a recording apparatus according to the invention. As shown in the figure, the primary components of this inkjet printer are a print unit 203 including a drive motor 202 for moving the inkjet head and the paper or other printed medium and an inkjet head 10, and control means for controlling the print unit. This inkjet printer prints text and/or graphics by ejecting ink to the paper or print medium from inkjet head 10 while moving inkjet head 10 and the print medium by means of drive motor 202. Timer means 204 counts the time. Nozzle recovery means 206 controls a process for recovering nozzles from clogging. A print operation controller 210 controls printing and various other operations to be executed in response to an input signal from input means 207, and outputs an initialization signal for starting timer means 204 and print control signals controlling print unit 203. The data used in the operations executed by print operation controller 210 are stored in storage means 211. A residual charge eliminator 212 outputs an actuator reset control signal for the reset process removing the residual charge in the actuator as described below. The configuration of drive control circuit 213 for inkjet head 10 is shown in Fig. 9. A recovery control signal from nozzle recovery means 206, a print control signal from the print operation controller 210, and the actuator reset control signal are input to drive control circuit 213, which controls inkjet head 10 based on these input control signals. The recovery control signal, print control signal, and actuator reset control signal are also input to a drive control circuit 214 for drive motor 202, and drive control circuit 214 similarly controls drive motor 202 based on these input control signals. Fig. 9 is a schematic diagram of the drive control circuit 213. As shown in the figure, drive control circuit 213 comprises a control circuit 215 and a drive circuit 102a. Drive circuit 102a comprises transistors 106 - 109, inverting amplifiers 110 and 108, and non-inverting amplifiers 111 and 113. The recovery control signal, print control signal, and actuator reset control signal are input into control circuit 215, which generates and outputs appropriate pulse voltages P1 - P4 for output to amplifiers 110 - 113 based on the input control signals. Transistors 106 - 109 are driven by the outputs from amplifiers 110 - 113, thus charging and discharging the capacitor 114 formed by diaphragm 5 and nozzle electrode 21 to emit ink droplets 104 from nozzle 4. By appropriately selecting the resistance values of resistors 115 and 116 desired charge/discharge characteristics may be obtained such as a relatively low charge speed and a relatively fast discharge speed. Fig. 10 shows an overview of a printer as an example of an inkjet recording apparatus that incorporates the inkjet head described above. 300 denotes a platen as a paper transport means that feeds recording paper 105 and is driven by drive motor 202 (Fig. 1). 301 indicates an ink tank that stores ink in it and supplies ink to the inkjet head 10 through an ink supply tube 306. The inkjet head 10 is mounted on a carriage 302 which is movable by means of carriage drive means (not shown) including drive motor 202 in a direction perpendicular to the direction in which the recording paper 105 is transported. To prevent or recover the nozzles from clogging, in response to the recovery control signal, the inkjet head is moved to a position in front of a cap 304, and then ink discharge operations are performed several times while a pump 303 is used to suction the ink through the cap 304 and a waste ink recovery tube 308 into a waste ink reservoir 305. Fig. 11 is a flow chart illustrating a method of controlling the inkjet printer according to the preferred embodiment of the invention shown in Fig. 1. Fig. 12 is a flow chart of two subroutines shown in Fig. 11, Fig. 12 (a) being the nozzle recovery operation subroutine and (b) the print operation subroutine. Referring to Fig. 11, the first step so is to initialize the printer mechanisms based on the control signals output from print operation controller 210. For example, as a result of the initialization, the carriage 302 (Fig. 10) is located at a standby position. Timer means 204 is simultaneously reset and begins counting the time. At step S1, the nozzle recovery operation is executed immediately after the power is turned on. This nozzle recovery operation executes steps SS1 - SS3 in the nozzle recovery subroutine shown in Fig. 12 (a), and is described below. At step SS1, carriage 302 carrying inkjet head 10 is moved from the standby position to the position of cap 304 by driving drive motor 202. At step SS2, the nozzle recovery operation is executed. This nozzle recovery operation drives the actuators of all nozzles to eject a predetermined amount of ink from all nozzles to remove dried or concentrated (high viscosity) ink, which otherwise could cause ink ejection defects, from the nozzles of inkjet head 10. A number of 10 - 200 ink droplets is normally ejected from each nozzle to expel any residual ink from the nozzles. After that the carriage 302 is returned to the standby position (step SS3) to complete the nozzle recovery operation. The frequency at which this recovery operation is repeated is determined by the time setting of timer means 204. Note that, in general, the inkjet head has not been used for an extended period of time when the power is first turned on, and about 160 - 200 ink droplets are ejected from each nozzle for nozzle recovery in step S1. When the nozzle recovery operation is completed, timer means 204 begins counting a predetermined time. A timer up signal is checked at step S2 to determine whether timer means 204 has counted the predetermined time. If the timer up signal is detected, the procedure flows to step S8, at which the nozzle recovery operation shown in the Fig. 12 (a) subroutine is again executed, and the procedure then advances to step S3. If, however, the timer up signal is not detected, the procedure flows directly to step S3. At step S3 it is determined whether to proceed with printing. If printing is not required, the procedure loops back to step S2. If printing is required, timer means 204 is reset in step S4, and the printing operation is executed in step S5. This printing operation is controlled by the subroutine of steps SS10 - SS16 shown in Fig. 12 (b). At step SS10 a count variable n is reset to 1, and then carriage 302 is moved one dot (step SS11). In steps SS12 and SS13, ink is suctioned and ejected at the specified dot based on printing data. After that the actuator reset operation is executed in step SS14, and then the count variable n is incremented to n+1. Equality of n to the number of the last dot to be printed is determined in step SS16. If n does not equal the last dot number, the procedure loops back to step SS11, and steps SS11 - SS16 are repeated. Note that, the actuator reset operation in step SS14 is executed only on those actuators which were driven in the preceding steps SS12 and SS13. If n equals the last dot number, the procedure exits the subroutine and advances to step S6, at which point carriage 302 is returned to the standby position, and the paper is then advanced a predetermined distance (step S7). Whether the process is to continue is evaluated in step S9; if printing is not completed, the procedure loops back to step S2 and the above operation is repeated. If printing is completed, the procedure terminates. Fig. 13 is a timing chart of the operation of the embodiment illustrated in Figs. 9 and 12. It is assumed here that, in the standby state, pulse voltage P4 is applied and transistor 108 is ON thereby keeping the capacitor 114 discharged via a resistor R. Then, to print a dot first of all pulse voltages P1 and P4 are supplied, transistors 108 and 107 become ON, and a positive voltage is applied to diaphragm 5 and nozzle electrodes 21 via a charge resistor 116 during period a. This causes a forward charge to accumulate in capacitor 114. Diaphragm 5 thus deflects toward nozzle electrode 21 due to the resulting electrostatic attraction force, the pressure inside ink chamber 6 drops, and ink 103 is supplied from ink cavity 8 through orifice 7 to ink chamber 6. After waiting for a hold period b, pulse voltages P2 and P4 are supplied, transistors 106 and 108 become ON shorting the capacitor 114 via a discharge resistor 115, and the charge stored in capacitor 114 is rapidly discharged. The electrostatic attraction force acting between diaphragm 5 and nozzle electrode 21 thus disappears, and diaphragm 5 returns due to its inherent rigidity. . Return of diaphragm 5 rapidly increases the pressure inside ink chamber 6, causing ink droplet 104 to be ejected from nozzle 4 toward recording paper 105. As indicated in period d, to achieve an actuator reset pulse voltages P2 and P3 are supplied, transistors 106 and 109 become ON, and a voltage of such polarity is applied to diaphragm 5 and nozzle electrode 21, that the diaphragm becomes negative with respect to the nozzle electrode. Note that this voltage is opposite to the voltage applied during the normal printing operation, and is opposite to the residual charge voltage. As a result, the residual charge (Fig. 7) dissipates. Diaphragm 5 is no longer deflected as shown in Fig. 8 (c) but is fully restored by discharging the capacitor during period e because the residual charge has been completely dissipated by the previous application of the reverse voltage as described above. Thus, the ink ejection volume that will be ejected at next period c2 and that at the previous period c are the same. As thus described, the residual charge created between diaphragm 5 and nozzle electrodes 21 is discharged each dot after ejecting an ink droplet 104. Incidentally, the hold period b mentioned above is preferably set such that the discharge occurs when the damped vibration of the ink system reaches a maximum so as to effectively utilize the vibration energy of the ink system. It is to be noted that while a reverse (negative) voltage is applied in the preferred embodiment above to eliminate the residual charge, the reverse voltage will also deflect diaphragm 5, and it is necessary to prevent ink being ejected at this time. When a semiconductor is used for diaphragm 5, there is minimal deflection even when the reverse voltage equals the forward voltage, and there is thus no danger of ink being ejected by reverse voltage application. It is therefore possible to use a common power supply in this embodiment. When a conductor is used for diaphragm 5, however, ink may be ejected if the reverse voltage equals the forward voltage, and it is therefore necessary to reduce the reverse voltage. Note also that a p-type semiconductor is used for the semiconductor substrate in this embodiment, but that an n-type semiconductor can be alternatively used. In this case, the connections between drive circuit 102a and inkjet head 10 must be reversed from those used with a p-type semiconductor. Fig. 14 is a flow chart of an alternative control method for the inkjet printer of Fig. 1. Fig. 15 is a flow chart of the print operation subroutine shown in Fig. 14. In this embodiment, the actuator reset operation is executed once every line. The actuator reset operation described above is executed in step SS12 inserted between steps S4 and S5 in Fig. 14. Note that, the actuator reset operation of this embodiment is executed at once for all actuators of the inkjet head in order to eliminate the residual charge which accumulated during one line printing. As a result, the actuator reset operation (step SS12) in the printing operation subroutine shown in Fig. 12 (b) is eliminated from the printing operation subroutine (Fig. 15 ) of this embodiment, but all other procedure steps are the same. The nozzle recovery operation subroutine in this embodiment is the same as that shown in Fig. 12 (a). Fig. 16 is a timing chart of the operation of this embodiment described in Figs. 14 and 15. In this embodiment, pulse voltages P2 and P4 are supplied and transistors 106 and 109 become ON during period a, each time carriage 302 returns after having completed one line, thus applying a reverse voltage to diaphragm 5 and nozzle electrode 21 to eliminate the accumulated residual charge as described above. Fig. 17 is a flow chart of another alternative control method for the inkjet printer of Fig. 1. Fig. 18 is a flow chart of two subroutines shown in Fig. 17, Fig. 18 (a) being the nozzle recovery/actuator reset operation subroutine and (b) the print operation subroutine. In this embodiment, the actuator reset operation is executed at once for all actuators of the inkjet head at the same time as the nozzle recovery operation (steps S1a and S8a). Thus, steps S1a and S8a in Fig. 17 take the place of steps S1 and S8 in Fig. 11. As a result, in the subroutine shown in Fig. 18 (a), carriage 302 is moved to the standby position (step SS1), and the actuators are then reset in the next step (step SS12). Step SS12 (Fig. 12) is thus eliminated from the printing operation subroutine (Fig. 18 (b)) of this embodiment. According to the first embodiment described above the influence of the residual charge is avoided by periodically removing the residual charge, either once every printed dot, once every printed line or based on a time count. Incidentally, these alternatives of the first embodiment may also be combined. By removing the residual charge in this way, i.e. by resetting the electrostatic actuators into a defined state, even if the residual deflection cannot be avoided, it is at least made constant. The effect of a constant residual deflection can be easily compensated for by a correspondingly increased drive voltage. A second embodiment of the invention will now be described with reference mainly to Figs. 19 to 25. It is well known that the relationship between the dipole moment p of a molecule of a previously unipolar dielectric upon applying an electric filed E is given by p = α·E wherein α is the molecular electric polarizability. Referring to Fig. 7, the relationship P = ε·x·Emax can be defined, where P is the residual field, x may be called a residual polarizability, Emax is the maximum field strength in the applied field history, and ε is the dielectric constant in vacuum. As shown by this equation, the residual field P is determined by the maximum field strength (voltage) in the applied field history, and so are the charge of the residual field and the initial deflection of diaphragm 5 resulting therefrom. Fig. 20 shows the deflection of the diaphragm under various conditions. The initial zero-deflection state of diaphragm 5 with no voltage history is shown in Fig. 20 (a). Note, diaphragm 5 is straight and parallel to the nozzle electrode 21. When a voltage (30 V) is then applied to the capacitor comprising diaphragm 5 and nozzle electrode 21, diaphragm 5 deflects by ΔV1 as shown in Fig. 20 (b). When subsequently the capacitor is discharged, diaphragm 5 assumes the state shown in Fig. 20 (c) with a deflection of ΔV2. Because of the voltage history of the applied 30 V, the residual field produced by the residual charge remaining after discharge of the capacitor causes diaphragm 5 to deflect slightly from the initial state shown in Fig. 20 (a). The ink on diaphragm 5 is eliminated and the ink elimination volume is determined by the difference between the deflection of diaphragm 5 shown in Fig. 20 (b) and the deflection shown in Fig. 20 (c). The ink elimination volume contributes to ejecting the ink droplet, and the ink volume corresponds to the difference ΔV3 (relative displacement) (see Fig. 20 (b)) of the diaphragm 5 deflection in the various states. From the state shown in Fig. 20 (c), an even higher voltage (40 V) charge is then applied (Fig. 20 (d)) to again deflect diaphragm 5. Subsequently, a switch is switched over to discharge the capacitor, and diaphragm 5 assumes the state shown in Fig. 20 (e). Note that diaphragm 5 deflection (ΔV4) shown in Fig. 20 (e) is greater than that (ΔV2) shown in Fig. 20 (c) because the residual field produced by the residual charge after discharge from the 40 V supply is stronger than that after discharge from the 30 V supply. Thus the strength of the residual field depends on the maximum voltage value in the history of voltage supply, and diaphragm 5 deflection accordingly also depends on it. Fig. 20 (f) shows the diaphragm 5 deflection when the same voltage (30 V) applied in Fig. 20 (b) is again applied after Fig. 20 (e). The diaphragm 5 deflection at this time is the same as shown in Fig. 20 (b) ΔV1). In this case, however, the ink elimination volume determined by the relative displacement corresponds to ΔV5 = ΔV1 - ΔV4 , which is determined by the difference between the Fig. 20 (e) deflection and the Fig. 20 (f) deflection, because the maximum voltage value in the history of voltage supply is 40 V. Note that ΔV3 > ΔV5. Therefore, the ink ejection volume when the inkjet head is driven in the state shown in Fig. 20 (f) where the maximum voltage of the voltage history is 40 V is less than that when the inkjet head is driven with a maximum voltage history value of 30 V as shown in Fig. 20 (b). We therefore know that the ink ejection volume varies with the level of the residual charge in the actuator comprising diaphragm 5 and nozzle electrodes 21. Fig. 21 illustrates experimental results of how the ink ejection speed at a constant 38 V drive voltage varies relative to the drive voltage applied in the preceding period. The ink ejection speed (1) was measured after driving the inkjet head for 10 min at a constant drive voltage of 38 V. The ink ejection speeds (2), (3) and (4) were measured after driving the inkjet head for 10 min at a constant drive voltage of 39 V, 40 V and 41 V, respectively, and then switching the drive voltage to 38 V. Note that when the experiments started the actuator had no residual charge (condition of Fig. 20 (a)), the driving frequency was 3 kHz and a charge pulse width of 30 µs was used. The ink ejection speeds (1), (2) , (3) and (4) are approximately 4 m/sec., 3.3 m/sec., 2.8 m/sec., and 1 m/sec., respectively. As this illustrates, even when the drive voltage remains constant, the ink ejection speed varies according to the magnitude of the drive voltage applied in the preceding period. The cause of this is the residual charge described above. This change in the relative displacement of diaphragm 5 and nozzle electrode 21 causes a change in the ink ejection speed and ink ejection volume, and thus adversely affects inkjet printer reliability and print quality. To counter this in the second embodiment, a maximum voltage is applied between diaphragm 5 and nozzle electrode 21 to maintain a maximum constant residual charge and to predetermine an initial diaphragm 5 deflection and also to stabilize the ink ejection speed and volume (this may be called a presetting of the actuator). If a 41 V maximum voltage is applied as the first drive voltage and the drive voltage then applied is, for example, 39 V or 40 V, the ink ejection speed at a 38 V drive voltage will be determined by the difference in diaphragm 5 deflection at a 38 V drive voltage and the deflection caused by the residual charge of the 41 V drive voltage, and will be unconditionally constant and stable. A block diagram of an inkjet printer according to the second embodiment of the present invention is shown in Fig. 19. This inkjet printer further comprises a power supply voltage adjustment means 412 and uses a drive control circuit 413 which is different from the drive control circuit 213 of the first embodiment. Power supply voltage adjustment means 412 appropriately selects and outputs the normal printing drive voltage Vn or a maximum voltage Vm imparting the voltage history of a known maximum voltage (where Vm > Vn) for presetting the actuator for the reasons explained above. Note that, the maximum voltage Vm should be determined by considering a tolerance of the power supply voltage, for example, when the range of the normal printing drive voltage Vn is 30 V ±10%, the maximum voltage Vm may be more than 33 V at least. Drive control circuit 413 controls inkjet head 10, and is constructed as shown in Fig. 22. The recovery control signal, print control signal, and drive voltage Vn or Vm are input to drive control circuit 413, which controls inkjet head 10 based on these control signals. Other components and functions of the printer shown in Fig. 19 are the same as those of the printer shown in Fig. 1, and further description is therefore omitted below. Fig. 22 is a schematic diagram of drive control circuit 413 for inkjet head 10. As shown in the figure, drive control circuit 413 comprises control circuit 415 and drive circuit 102b. The recovery control signal and print control signal are input to control circuit 415, which outputs charge signal 51 and discharge signal 52 based on these input control signals. Drive circuit 102b comprises transistors 41, 42, 44, and 45. When drive control circuit 413 is in the standby mode, transistors 42 and 45 are both OFF, and the drive voltage is not applied to diaphragm 5 and nozzle electrode 21. Diaphragm 5 is therefore not displaced, and no pressure is applied to the ink in ink chamber 6. When charge signal 51 becomes ON, transistor 41 becomes ON at the charge signal 51 rise, and transistor 42 also becomes ON. The drive voltage Vn (or maximum voltage Vm) is therefore applied between diaphragm 5 and nozzle electrode 21 via charge resistor 43, current flows in the direction of arrow A, and diaphragm 5 is deflected towards nozzle electrode 21 by the electrostatic force working between diaphragm 5 and nozzle electrode 21 due to the charge accumulated therebetween. The volume of ink chamber 6 is thus increased, and ink is suctioned into ink chamber 6. When charge signal 51 becomes OFF and discharge signal 52 becomes ON, both transistors 41 and 42 become OFF, and charging between diaphragm 5 and nozzle electrodes 21 stops. Transistor 44 also becomes OFF, and transistor 45 becomes ON as a result. When transistor 45 is ON, the charge accumulated on diaphragm 5 and nozzle electrode 21 is discharged in the direction of arrow B through a discharge resistor 46. Because the resistance of resistor 46 is significantly lower than the resistance of the charge resistor 43 and the time constant of the discharge is low in this embodiment, the accumulated charge can be discharged in sufficiently less time than the charge time. Diaphragm 5 is immediately released from the electrostatic force at this time, and returns to the non-printing standby position due to the inherent rigidity of the diaphragm material. This rapidly compresses ink chamber 6, and the pressure produced inside ink chamber 6 causes ink droplet 104 to be ejected from nozzle 4. It is to be noted that while a p-type semiconductor is used as the substrate in this embodiment, an n-type semiconductor can be alternatively used. In this case, the connections between drive circuit 102b and inkjet head 10 must be reversed from those used with a p-type semiconductor. Fig. 23 is a flow chart of the inkjet printer control method for the embodiment of the invention shown in Fig. 19. In this embodiment, a high voltage is applied after executing the initialization routine. The first step S0 is to initialize the printer mechanisms based on the control signals output from print operation controller 210. Timer means 204 is simultaneously reset and begins counting the time, and carriage 302 carrying inkjet head 10 is moved from the standby position to the position of cap 304 by driving drive motor 202. At the next step S10, power supply voltage adjustment means 412 selects and outputs the maximum voltage Vm to drive control circuit 413 of inkjet head 10. The print control signal is input from print operation controller 210 to control circuit 415, which sequentially outputs charge signal 51 and discharge signal 52 to drive circuit 102b. The maximum voltage Vm is thus applied between diaphragm 5 and nozzle electrode 21 of all actuators, imparting the voltage history of maximum voltage Vm to the dielectric between diaphragm 5 and nozzle electrode 21, and one ink droplet, for example, is released from all nozzles. Power supply voltage adjustment means 412 then resets the output voltage to the normal print operation drive voltage Vn. The nozzle recovery operation immediately after the power is turned on is then executed at step S1. This nozzle recovery operation executes steps SS1 - SS3 in the nozzle recovery operation subroutine shown in Fig. 12 (a). This subroutine is as described above, and further description is therefore omitted. After completing the nozzle recovery operation, timer means 204 begins counting a predetermined time. A timer up signal is checked at step S2 to determine whether timer means 204 has counted the predetermined time. If the timer up signal is detected, the procedure flows to the nozzle recovery operation (step S8), the nozzle recovery operation shown in the Fig. 12 (a) subroutine is again executed, and the procedure then advances to step S3. If, however, the timer up signal is not detected, the procedure flows directly to step S3. At step S3 it is determined whether to proceed with printing. If printing is not required, the procedure loops back to step S2. If printing is required, timer means 204 is reset in step S4, and the printing operation is executed in step S5. This printing operation is controlled by the subroutine of steps SS10 - SS16 shown in Fig. 15 (b). During this printing operation ink droplets are ejected by successively supplying the charge signal 51 to turn transistors 41 and 42 ON, and then the discharge signal 52 turning transistors 44 and 45 ON as mentioned before. The residual field at this time is dependent upon the voltage history of the past maximum voltage Vm, and diaphragm 5 is therefore shows a slight residual deflection, but the residual charge remains constant irrespective of the drive voltage history even if the drive voltage varies within the range up to the maximum voltage Vm. After the last dot has been printed the procedure exits the subroutine and advances to step S6. Steps S6, S7 and S9 are the same as in Figs. 11 and 14. Fig. 24 is a flow chart of an alternative control method for the inkjet printer of the second embodiment of the invention shown in Fig. 19. Fig. 25 is a flow chart of two subroutines shown in Fig. 24, Fig. 25 (a) being the nozzle recovery operation subroutine and (b) the print operation subroutine. In this embodiment, a high voltage is applied during the nozzle recovery operation, and is specifically applied when the nozzles are recovered by the nozzle recovery operation shown in steps S1b and S8b in Fig. 24. At step SS1 (Fig. 25 (a)), carriage 302 carrying inkjet head 10 is moved from the standby position to the cap 304 position by driving drive motor 202. At step S10, the maximum voltage Vm is applied as the drive voltage as described above to eject one ink droplet 104 from all of the nozzles. The normal printing drive voltage Vn is then applied, and the nozzles are recovered in steps SS2, SS3. It is to be noted that while maximum voltage Vm application is separated from the nozzle recovery operation in this embodiment, step S10 in Fig. 25 (a) can be omitted and the maximum voltage Vm applied during the nozzle recovery operation of step SS2.

Claims: An inkjet recording apparatus comprising an inkjet head (10) having for each of one or more nozzles (4), an ink passage (6, 7, 8) in communication with the nozzle, and an electrostatic actuator (5, 21, 27) comprising a pair of capacitor plates (5, 21) of which one plate is formed by or attached to a diaphragm (5) provided in a part of said ink passage, while the other plate is formed by an electrode (21) disposed outside of said ink passage in opposition to the diaphragm with a gap (G) therebetween, and drive means (210, 212, 213; 210, 412, 413) responsive to a recording control signal for selectively charging each actuator (5, 21, 27) by applying a first voltage to said capacitor plates of a respective actuator and subsequently discharging it, such as to displace its diaphragm (5) by an electrostatic force, thereby to eject ink droplets from said one or more nozzles (4), characterized in that said drive means comprises set/reset means (212; 412) responsive to another control signal for setting or resetting an actuator by applying a second voltage different from said first voltage. The apparatus according to claim 1, wherein said set/reset means (212) is adapted to apply as said second voltage one whose polarity is opposite to that of said first voltage and whose magnitude is selected such as to reset the actuator (5, 21, 27) by removing a residual charge. The apparatus according to Claim 2, wherein said set/reset means (212) is adapted to apply said second voltage to an actuator (5, 21, 27) following each charge/discharge cycle of that actuator or to apply said second voltage simultaneously to each of said one or more actuators after one line of recording has been completed. The apparatus according to Claim 2 or 3, further comprising means (206) for performing a nozzle recovery by driving each of said one or more actuators (5, 21, 27) to eject one or more ink droplets, wherein said set/reset means is adapted to apply said second voltage when the nozzle recovery operation is executed. The apparatus according to claim 1, wherein said set/reset means (412) is adapted to apply as said second voltage a voltage equal to or greater than the maximum of said first voltage. The apparatus according to Claim 5, further comprising means (206) for performing a nozzle recovery by driving each of said one or more actuators (5, 21, 27) to eject one or more ink droplets, wherein said set/reset means (412) is adapted to apply said second voltage during the nozzle recovery operation or during an initialization operation of the apparatus. The apparatus according to Claim 5 or 6, wherein said second voltage is equal to or greater than 1.1 times said first voltage. A method of driving one or more electrostatic actuators (5, 21, 27) in a recording apparatus as defined in claim 1, comprising the steps of: (a) selectively charging, in response to recording data, said one or more actuators (5, 21, 27) by applying a first voltage, and (b) subsequently discharging each actuator charged in step (a) for causing ink ejection, (c) applying to one or more of said actuators a second voltage different from the first voltage and subsequently discharging it. The method according to claim 8, wherein step (c) comprises applying a voltage of a polarity opposite to that of said first voltage. The method according to claim 9, wherein either each time an actuator (5, 21, 27) has undergone steps (a) and (b), step (c) is executed with respect to this actuator, or step (c) is simultaneously executed with respect to each of said one or more actuators each time recording of one line has been completed. The method according to claim 9 or 10, further comprising a nozzle recovery step, wherein step (c) is executed each time the nozzle recovery step is executed. The method according to claim 8, wherein step (c) comprises applying a voltage equal to or greater than the maximum of said first voltage. The method according to claim 12, further comprising an initialization step for initializing the recording apparatus and a nozzle recovery step, wherein step (c) is executed when the initialization step is executed and/or each time the nozzle recovery step is executed. The method according to claim 12 or 13 wherein step (c) comprises applying a voltage equal to or greater than 1.1 times that of said first voltage.

Inventors: FUJI MASAHIRO, KOEDA HIROSHI, MIYASHITA IKUHIRO, FUJI, MASAHIRO, KOEDA, HIROSHI, MIYASHITA, IKUHIRO, FUJI, MASAHIRO, C/O SEIKO EPSON CORPORATION, KOEDA, HIROSHI, C/O SEIKO EPSON CORPORATION, MIYASHITA, IKUHIRO, C/O SEIKO EPSON CORPORATION
Applicants: SEIKO EPSON CORP, SEIKO EPSON CORPORATION
Application year: 1994
Main classification: B41J2
IPC codes: B41J2